This research program is concerned with the neuronal and synaptic organization of the mammalian somatosensory thalamus. This region of the brain receives afferent information from ascending somatosensory systems and transmits this information to the somatosensory cortex. Although the thalamus is frequently termed a "relay nucleus", it is evident that there is much complex information processing within this major diencephalic structure. The neuronal circuitry of the thalamus is being examined by several experimental techniques: 1) orthograde transport of wheat germ lectin - microinjections of lectin conjugated to horseradish peroxidase (WGA-HRP) are made into selected regions of the neuraxis which are known to project to the somatosensory thalamus. Utilizing a newly developed methodology, the synapses arising from various sites which project to the thalamus can be revealed by electron microscopy and their precise distribution over the surfaces of thalamic neurons determined. 2) Intracellular labeling of physiologically characterized somatosensory thalamic neurons combined in some cases with the axon transport methodology can reveal the synaptic populations which contact these neurons, as well as the projections of particular pathways upon physiologically-characterized somatosensory neurons. The synaptic input to these neurons is then mapped using a three-dimensional computer reconstruction program. 3) Immunocytochemical methods are used to describe those neuronal and synaptic structures which contain putative neural transmitters, such as gamma-aminobutyric acid (GABA), serotonin, acetylcholine or norepinephrine. It is believed that some of these transmitters change the firing patterns of thalamic neurons and thus alter the transmission of information through the thalamus 4) Monoclonal antibodies have been raised against synaptosome fractions obtained from somatosensory thalamus and subsequently screened for their activity. We have obtained six antibodies (out of more than 3,000) which appear to selectively label particular elements within the somatosensory thalamus. These antibodies will be examined to determine whether they can be used to identify certain classes of axon terminals within the thalamus, in order to further expand our abilities to analyze thalamic circuitry. The combined use of modern anatomical and physiological methods to be used in this study provide a powerful means to relate structural and functional properties of neurons which constitute a major element of the nervous system concerned with a variety of somatosensory mechanisms, which allow animals and humans to detect and interpret stimuli arising from cutaneous and deep tissues of the body.